Where an organic polymer having a silicon-containing hydrolyzable group is used as a one-component curable composition, it is known that the storage stability of the composition can be improved by addition of a low-molecular silicon compound having a silicon-containing hydrolyzable group which exhibits higher reactivity to H.sub.2 O than the silicon-containing hydrolyzable group of the organic polymer.
The amount of the low-molecular silicon compound to be added for assurance of storage stability has conventionally been decided only by the H.sub.2 O content in the composition. However, when the conventional composition is actually used, the low-molecular silicon compound remaining in the composition adversely affects the curing properties of the composition. For example, curing is retarded, or a cured product becomes too hard.